I think what most hardware shops or building supplies sell as marine ply is WBP, (water boil proof) despite what people may say this isn't marine ply and does fall to bits upon prolonged imersion.
we made a trailer bed out of 3/4 WBP ply and varnished it well and it didnt last 2 years. it was replaced with the proper trailer deck board which is a tough resin faced non slip marine ply and it outlasted the rest of the trailer

I think what most hardware shops or building supplies sell as marine ply is WBP, (water boil proof) despite what people may say this isn't marine ply and does fall to bits upon prolonged imersion.
we made a trailer bed out of 3/4 WBP ply and varnished it well and it didnt last 2 years. it was replaced with the proper trailer deck board which is a tough resin faced non slip marine ply and it outlasted the rest of the trailer

I never knew that! So when it's badged up the same, how do you know which is which?

dont by marine ply from a builders merchant
Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine plywood. There are few international standards for grading marine plywood and most of the standards are voluntary. Some marine plywood has a Lloyd's of London stamp that implies that it is BS 1088 compliant. Some plywood is also labeled base on the wood used to manufacture it. If is is stamped WBP then its exterior grade and will fall to bits in salt water.